Tips on Preparing Concrete Slabs for Hardwood Floor Installation

Installing a wood floor over concrete

To many hardwood flooring contractors, installing hardwood floors over concrete can be a bit of a puzzle. The following is a guide to help alleviate any concerns or worries about installing hardwood floors over concrete.

The secret behind a successful install over concrete is to focus on five key areas: Condition of slab, type of concrete, location of slab, flatness, and moisture.

Read on for in-depth details on these five key areas:

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CONDITION OF THE CONCRETE SLAB

One of the easiest and most beneficial steps to take is to carefully examine the condition of the concrete slab. Take a look at the slab and note any cracks, spalling, or curling at expansion joints.

This is also the time to note any high or low spots in the concrete. Currently the National Wood Flooring Association (NWFA) recommends 1/8″ to 3/16″ in a 10FT radius. You may have to grind down the floor or use a self-leveling compound like Bostik SL-150 to even out the slab.

If you take the time to examine the concrete and set about to fix any potential problems, you will save yourself time and energy later on. After all, if concrete slab is in poor condition, your hardwood floors are more likely to fail.

MOISTURE CONTENT OF THE SLAB

Checking the moisture rating of a concrete slab is arguably the most important test to perform when assessing a concrete slab. Moisture is said to cause upwards of 90% of hardwood flooring failures.

Watch our video on using the Wagner Rapid RH 4.0 to measure the RH of a concrete slab.

WATCH: How to Test the Relative Humidity of a Concrete Slab using the Wagner Rapid RH

When it comes to moisture and concrete slabs the two most recognized tests are calcium chloride and relative humidity. Click here to learn more about these two tests, and how they compare.

Understanding the moisture content of a concrete slab and taking the necessary steps to combat moisture will help immensely in successfully installing hardwood floors over concrete.

TYPE OF CONCRETE

We know that there are many different types of hardwood that perform differently. We pay attention to the type of wood and work with it in a way to get the best possible installation. The same is true with concrete.

There will be information (provided by architects, builders, and engineers) that gives specific information about the concrete slab in question. This information will include figures like PSI, CSP (Concrete Surface Profile), and Flatness (FL, FF, & F-min). Sometimes you’ll find the concrete is actually a lightweight concrete. In that particular scenario, you’ll have to take additional steps before installing the hardwood floors. If you didn’t know a concrete slab was lightweight concrete, you might have incorrectly installed the hardwood floor.

Gathering the information on the concrete will aid you during the install. You’ll be on the path to correctly install the hardwood floor on the specified concrete.

LOCATION OF SLAB

Above grade vs. below grade for concrete slab

Image credit www.hardwoodinfo.com

This step might be the easiest to determine. However, there are still things you need to take note of. When determining the location of the slab what you’re looking for is whether the slab is at, above, or below grade.

No matter where the concrete slab is located, you must know how to deal with it. Slabs above grade have less moisture than below grade slabs. However, slabs above grade might also have a weight limit. For example in a condominium you might not be able to put down leveling compound because of the weight it would add to the concrete slab. In a below grade install there is more moisture and usually that means adding moisture/vapor barriers.

FLATNESS

In most scenarios when a concrete slab is poured, the concrete is poured and screeded into a pre-made form. The professionals pouring the concrete will surely make the slab seems level and flat, but that does not necessarily mean the concrete is flat enough to install hardwood floors.

Using one of many different tools — ten foot straight edge, mason’s string, or a laser — can determine the true flatness of a concrete slab. Typically a floor is flat when it is 3/16″ over a 10′ radius or 1/8″ over a 6′ radius.

If the concrete slab is not level then you either need to use leveling compound to fill in low spots or grind away high spots. Click here to learn more about the Bostik SL-150 self-leveling compound.

Or, if the situation requires it, installing a subfloor over the concrete slab might be the best option.

In any scenario, making sure the concrete is flat and level enough to install hardwood floors over is paramount.


Hardwood floors are installed over concrete by wood flooring mechanics all the time. When you are knowledgeable about how concrete works and how it interacts with hardwood flooring, you will be prepared to install a beautiful floor over concrete.

If you’d like to learn more about preparing a concrete slab for hardwood floors, call us at (800) 787-1786 or email info [at] cityfloorsupply.com

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8 thoughts on “Tips on Preparing Concrete Slabs for Hardwood Floor Installation

  1. Harper Campbell

    It’s interesting to learn when it comes to installing hardwood floors over concrete that one thing we need to look at beforehand is what kind of concrete we have. I am glad that you mentioned we need to know if the concrete is lightweight so we will know if there are more steps we need to do before installing the hardwood. This is something we will have to remember when it comes to finishing our basement.

    Reply
  2. Sam Solo

    I didn’t know that levels that are above grade have less moisture. I guess this would make sense since the soil captures more water along with any possible root systems. I imagine you would have to be very careful about how deep you build the grade so that the house doesn’t develop mold.

    Reply
    1. Caran Baxter

      Typically on grade slabs are constructed with a layer of polyethylene film under them to prevent any moisture from wicking up through the slab.

      Reply
  3. Michael Robinson

    Thanks for teaching me more about preparing my concrete for some hardwood installation. As you mentioned, you should look into the condition of the concrete slab that is underneath and make sure that there are no cracks, spalling, or curling at expansion joints. My wife has wanted hardwood in our little home for years now, and I want to make sure that our home would qualify for hardwood. Thanks again, and I will contact a contractor as soon as I can.

    Reply
  4. Lloyd Concrete

    I’m excited to see your new website and its design. It’s been a while since I’ve been reading this blog. I would suggest that you redirect this domain to the new one.

    Reply
  5. Anonymous

    Thanks for sharing such nice information about the five key factors before tips on installing hardwood floor over concrete. Among these 5 factors as per my knowledge measurement of the humidity of the concrete slab is the most important one since moisture can be a great barrier for successful installation of hardwood floor. During the installation of the hardwood floor, it is always recommended to use no water adhesive glue. So from this, it can be easily understood that a lower level of moisture is always good for hardwood flooring. The two tests mentioned for detecting moisture is also very useful in this case.

    Reply

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